Two-stage dehydrogenation process for producing diolefins

ABSTRACT

A two-stage dehydrogenation process for producing diolefins from monoolefins wherein the monoolefin stream is first dehydrogenated under non-oxidative conditions, followed by an oxidative dehydrogenation with an iron phosphate catalyst. Isoprene is produced from a feedstream containing isoamylenes by a two-stage dehydrogenation process wherein the necessity of removing hydrogen from the non-oxidative effluent is eliminated through a selective second stage oxidative dehydrogenation stage.

United States Patent my Ripley 1 Apr. 3, 1973 [541 TWO-STAGE DEHYDROGENATION PROCESS FOR PRODUCING- DIOLEFINS [75] Inventor: Dennis L. Ripley, Bartlesville, Okla.

[73 l Assignee: Phillips Petroleum Company, Bartlesville, Okla.

[22] Filed: Aug. 2, 1971 [21] Appl. N0.: 168,464

[52] US. Cl. ..260/680 E, 260/666 A [51] Int. Cl ..C07c 5/18 [58] Field of Search ..260/680 E, 681.5 R

[56] References Cited UNITED STATES PATENTS 2,866,790 12/1958 Pitzer ..260/680 X 3,161,670 12/1964 Adams et a1 ..260/680 X 8/1966 Christmann ..260/680 Primary Examiner-Paul M. Coughlan, Jr. Att0meyJ. Arthur Young et a1.

[57] ABSTRACT A two-stage dehydrogenation process for producing diolefins from monoolefins wherein the monoole'fin stream is first dehydrogenated under non-oxidative conditions, followed by an oxidative dehydrogenation with an iron phosphate catalyst. lsoprene is produced from a feedstream containing isoamylenes by a twostage dehydrogenation process wherein the necessity of removing hydrogen from the nonoxidative effluent is eliminated through a selective second stage oxidative dehydrogenation stage.

6 Claims,'N0 Drawings TWO-STAGE DEHYDROGENATION PROCESS FOR PRODUCING DIOLEFINS This invention relates to the two-stage dehydrogenation of monoolefins to diolefins, more particularly isoamylenes to isoprene. In another aspect, this invention relates to a catalytic, non-oxidative dehydrogenation of an isoamylene-containing feedstream wherein the total effluent of the first stage is oxidatively dehydrogenated with an iron phosphate catalyst in a second stage. Another aspect of this invention is the two-stage dehydrogenation process wherein the molecular hydrogen present, as a result of the dehydrogenation reaction of the first stage, is not removed from the continuous feed to the second stage, nor is it substantially burned in the oxidative dehydrogenation conditions of the second stage.

The process of my invention finds particular utility in the conversion of isoamylenes to isoprene. Accordingly, the invention will generally be discussed hereafter as it relates to the aforementioned process.

In the catalytic dehydrogenation of isoamylenes to isoprene, two separate dehydrogenation equilibria and two different reaction rates are normally encountered. In general, it is found that at operating conditions where efficient and selective dehydrogenation of isoamylenes to isoprene is obtained, the yield of isoprene is low because of the limitation established by the thermodynamic equilibrium. On the other hand, at conditions which would thermodynamically favor high yields of diolefins, the primary dehydrogenation of the olefin to the diolefin is far too severe, resulting in an unduly high yield of cracked gases with poor dehydrogenation reaction selectively.

Straight dehydrogenation processes are limited by the aforementioned thermodynamics, to relatively low conversions. However, low conversions may be achieved with relatively good selectivity. An additional limitation is that the heat required by the reaction limits the extent of conversion obtainable in an adiabatic reactor, due to the temperature decrease. In the oxidative dehydrogenation process, because of heat supplied by oxidation, high conversions can be achieved since the thermodynamic limitation allows higher conversion rates. However, disadvantages of the oxidative process are the cost of oxygen necessary and the conversion limitations placed on an adiabatic reactor system by the temperature rise accompanying high conversions.

Some of the disadvantages of both non-oxidative and oxidative dehydrogenation stages can be alleviated by combining the two processes. For example, a conventional dehydrogenation catalyst can be employed in the first stage to give partial conversion with high selectivity and correspondingly small temperature decrease. This stage is followed by an oxidative dehydrogenation stage in which additional conversion is achieved. The combination of the two processes is particularly advantageous when a catalyst is used in the oxidative section which does not oxidize hydrogen to any appreciable extent. The overall process is benefited by the lower oxygen requirements, high conversion, and a less severe temperature profile in the reactor.

The present invention overcomes the aforementioned problems and permits a convenient and advantageous combination of the efficient non-oxidative dehydrogenation process with another equally efficient but oxidative dehydrogenation stage.

Accordingly, it is an object of the present invention to provide a two-stage dehydrogenation process wherein monoolefins are dehydrogenated to diolefins. Another object is to provide a process for the dehydrogenation of isoamylenes to isoprene. Yet another object is to provide a process having high diolefin yield conversion rates and good process efficiencies without the heretofore necessary purification steps.

The process of this invention pertains to a two-stage catalytic dehydrogenation production of diolefins from monoolefins. This process utilizes a non-oxidative dehydrogenation catalyst comprised of potassium carbonate, iron oxide, and chromium oxide for the firstphase and an iron-phosphate catalyst for the oxidative dehydrogenation of the second phase. Oxidative dehydrogenation of monoolefins to diolefins has been found to take place without the oxidation of free molecular hydrogen which is present from the firststage effluent when the iron-phosphate catalyst system is utilized. The total effluent of the first stage, including monoolefin, diolefin, hydrogen, and steam, can be directly passed to the second stage and dehydrogenated by oxidative dehydrogenation. A suitable amount of a molecular oxygen-containing gas is added to the mixture. prior to the second stage. If sufficient steam is not already present, a suitable amount of steam is also added to the second stage.

The hydrogen generated in the first-stage effluent does not react in the second stage to any appreciable extent, therefore not causing excessive heat release and temperature rise in the second-stage oxidative process. The hydrogen in the second-stage effluent can be effectively separated for other uses. Moreover, linear pentenes which might be present in the effluent of the first stage are not appreciably dehydrogenated to piperylene in the second stage which utilizes the ironphosphate catalyst system.

From the above statement of the invention, it is readily apparent that the production of diolefins from corresponding monoolefins can be effected in a manner considerably simpler and more efficient than heretofore disclosed by the art. Thus, in comparing the process of the present invention with that of conven tional two-stage operations known to the art, it can be seen that the present process l eliminates the necessity of removing hydrogen from the first stage effluent and (2) is relatively unaffected by n-amylenes, for example, present as a result of dehydrogenation of normal pentane in a previous dehydrogenation stage. For example, n-pentenes are converted to only a slight extent and there is very little n-pentadiene formation, thus simplifying isoprene purification. A further advantage of the present process over conventional operations is that the effluent from the first-stage dehydrogenation zone passes in its entirety along with added oxygen and steam, directly to the secondary oxidative dehydrogenation zone, with no substantial change in temperature or pressure.

Among the many olefinic compounds which can be dehydrogenated in the instant process are butene-l, butene-2, pentene-l, pentene-Z, isoamylenes, 2- methyl-pentene-l, 3-methyl-pentene-l, Z-methyI-butene-2, hexene-l, hexene-2, 4-methyl-pentene4, 3,4- dimethyl-pentene-l, 4-methylpentene-2, heptene-l, octene-l cyclohexene, 3-methylcyclohexene, and

cyclopentene. The alkenes can contain from three to ten, preferably four to six, carbon stoms per molecule, inclusive, and the cycloalkenes can contain from four to ten, preferably four to six, carbon atoms per molecule, inclusive. Open chain olefins yield diolefins and, in general, six-membered ring olefms yield aromatic ring compounds, the higher molecular weight open-chain olefms may cyclicize to aromatic ring compounds.

Suitable apparatus known to the art using conventional modes for contacting feedstreams with the selected catalyst can be used in the process of this invention. The basic requirement of the primary dehydrogenation zone is that it converts, as efficiently as possible, the monoolefins into diolefins. Multi-tubular reactors and vessels containing catalyst beds are well known and have been successfully used by the art for such dehydrogenation processes.

The first stage dehydrogenation catalyst used in the present invention can be any of the conventional catalysts generally employed, for example, those containing metals of Groups IVB, VB, VIB, VIII, and xides of Group VIII metals, e.g., chromia on alumina, vanadia on alumina, nickel on kieselguhr, platinum on alumina, iron oxide on an organic carbonate, and the like. The conditions for the first-stage dehydrogenation zone may vary accordingly within the ranges stated above, depending upon the catalyst chosen.

The first-stage dehydrogenation process is conducted under a temperature range of from about 600 to 1,300F and under a conventional non-oxidative dehydrogenation zone pressure. A preferred catalyst for the first-stage dehydrogenation is an iron oxide catalyst containing a small amount of chromium oxide as a stabilizer and a small amount of potassium compound as a promoter. In particular, iron oxide should be present in an amount of about 39.0 to about 47.0 percent by weight and chromium oxide should be present in an amount of about L0 to about 10.0 percent by weight. Also, the potassium carbonate content of the catalyst should be within the range of bout 51.0 to about 59.0 percent by weight. Within these ranges, the catalyst composition yields a satisfactory selectivity under a suitable temperature requirement as discussed hereinabove, with resultant operating economics and improved catalyst life. The dehydrogenation of the first stage using the catalyst as described hereinabove is generally carried out in the presence of sufficient steam to provide a steam-to-hydrocarbon volume ratio in the range of about 0.111 to about 30:]. The total gas hourly space velocity (GHSV) of the hydrocarbon and steam will range from about 100 to about 50,000, preferably from about 500 to about 20,000, volumes of gas per volume of catalyst per hour. The catalyst will slowly lose some activity and will, therefore, periodically require regeneration by conventional means, for example, by contacting with steam-diluted air at elevated temperatures.

The total gaseous effluent from the first-stage dehydrogenation zone, consisting of unreacted isoamylenes, isoprene, hydrogen, and with small amounts of lighter gases and heavier polymerization products, is mixed with from about 0.1 to about 3.0 volumes of a molecular oxygen-containing gas per volume of hydrocarbon contained in the effluent. The

effluent is then passed to the secondary oxidative dehydrogenation zone. If sufficient steam is not already present, steam is added to provide a steam-to-organic feed volume ratio in the range of about O.l:l to about The organic feed space rate can be from about 50 to about 5,000, preferably from about 100 to about 2,500 GHSV. The second dehydrogenation stage is conducted at a temperature of from about 800 to about 1,200F in the presence of steam, oxygen and iron-phosphate catalyst system which has the aforementioned advantage of having a high selectivity for the dehydrogenation of isoamylenes and a significant lack of activity for dehydrogenation of n-amylenes', or for the oxidation of free molecular hydrogen.

The iron-phosphate catalyst of the oxidative secondstage dehydrogenation zone is an iron-phosphorus-oxygen catalyst such that the amount of phosphorus present is in excess of the stoichiometric amount required for the phosphorus to react in the form of phosphate ions (1 0( with all the iron in the catalyst. Thus, depending upon the valence of the iron, the catalyst has a phosphorus content higher than that calculated for the corresponding iron phosphate compound. The iron with the catalyst compositions can be ferric, ferrous, or ferroso-ferric and will phosphorus contents higher than that calculated for the corresponding compound containing stoichiometric amounts of phosphorus, as shown in the following Table I.

TABLE I Thus, these specific iron-phosphate catalysts are iron-phosphorus-oxygen compositions in which the phosphorus content is generally in the range of from about 1.0] to about 5 times, preferably 1.01 to about 2 times, the stoichiometric amount required to react, in the form of phosphate ions, with all of the iron present, and the atomic ratio of oxygen to phosphorus is in the range of3:l to 3.99911.

Except for the greater-than-stoichiometric quantity of phosphorus, the catalysts can be prepared in a manner of suitable ways, such as by the treatment of iron oxides, iron hydroxides, iron phosphates, or other iron salts with phosphoric acid or by the dry mixing of iron oxides or iron salts with phosphorus pentoxide, and the like. The catalyst of this invention can be used in the form of granules, mechanically formed pellets, or any other conventional form of catalyst. If desired, the catalyst can also be employed with suitable supporting or diluting materials such as silica, alumina, boria, magnesia, titania, zirconia, and the like.

These catalysts can be activated by conventional calcination in air at elevated temperatures and can be used for very long periods of time without reactivation or regeneration. However, if regeneration becomes necessary, it can be accomplished simply by stopping the flow of hydrocarbon feed and allowing the flow of the other components, namely the air and steam, to

have

continue for a sufficient period of time to restore a substantial amount of the catalytic activity.

Additionally, it is preferred to maintain the catalyst in a high state of activity by the continuous or intermitlighter gases, and some heavy polymerization products, passes into conventional recovery facilities to separate and recover the total isoprene content from the effluent. Any means accomplishing this is suitable for use tent addition of phosphorus-containing compounds to 5 in the present process. Unconverted isoamylenes can the catalytic zone to insure the h g be recycled to the appropriate stage. An isoprene stoichiometric level of phosphorus in that catalytic tream, ontainin lativ l mall amount of zone. This can be don by add n f very Small q piperylene, can be recovered for further purification. tities of Compounds Such as P P acid, The following examples illustrate the results of phosphorus pentoxide, or other organophosphorus operating th bj t process f r h t t compounds such as triorganophosphines t0 the fe d dehydrogenation of pure 2-methylbutene-2 to produce mixture. The rate of addition of Such p o p isoprene. The examples further illustrate the singletaining compounds is that ,which is sufficient to mainstage, id i d h d i f pure 2- tain the desired phosphorus level in the catalyst deh m Q f Comparative purposes E l Pehdihg p the amount of Phosphorus which might and 11, presented in table form, and the corresponding be lost from the Catalyst as measured y the amount of results therein are shown without the intent to limit the Phosphorus found in the Steam condensate from the scope of the invention. They do, however, demonstrate reactor effluent. the feasibility of the inventive process.

The effluent from the secondary dehydrogen i n Theresults as illustrated in Examples I and 11 fully zone, comprising isoamylene, isoprene, hydrogen, demonstrate the advancement provided to olefin EXAMPLE I One-stage non-oxidatlve dehydrogenation Catalyst (iron oxide-chromium oxide-potassium carbonate) Yield, moles/100 mole 2-MB-2 feed Temp. Sample Time 1 Conv. Yield Ci 3-MB-1 2-MB-1 .Z-MB-2 Isoprcnc Son the following table:

2-nicthylbutcne-2 GHSV. Nitrogen GHSV Steam (1HS\ Two-stage (non-oxldative and oxidative) dehydrogenation First stage catalyst (iron oxide-chromium oxide-potassium carbonate); second stage catalsyt (iron-phosphate-oxygen) Yield, moles/ moles 2-MB-2 iced Tcmp., Sample Time 1 F. Conv. Yield Mod. CO C C4 3.\IB--l Q-MB-l 2MB2 Isoprcnv 1 3:0 1, 035 07. 0 50. 7 83. 5 10. 00 0. 84 .2 23 4. 50 10. 06 16. 56 51-1. 00 .2 3:30 1, 035 07. 0 58. 1 85. 5 8. 22 0. 05 2 33 4. 50 10. 85 10. (i7 58. 00 3 4:00 1, 035 70.1 57. (i 82. l 12. 88 1.15 .2 28 4. 03 0. 52 15. 44 57. 63 4 4:30 1,035 70.6 57.5 81.4 14.57 1.12 2 41 4. 43 0.70 15.24 47.43

1 Times are in hrs.:min. total time on stream. Hydrocarbon (il1S\' 400 2 Modivity is a modified selectivity. As used licrvin, the terms yield, ()xygnn GHSV. 300 conversion, and modivity" are based on analyses of gas-phase products Nitrogmn... 3,004 Steam 5, 000

for convortcd hydrocarbons, oxides of carbon, and nnconvnrtcd feed.

Son the following table:

The feed for the second stage is tliotffin ciit from the first stsiizc after water rr'moval and air and strain addition.

EXAMPLE l1 Ono-stage non-oxidativc dohydro zmiation Catalyst. (iron oxide-chromium ()Xi(ll'-]l(i1.i1$$1l.1lll r-nrbonatc) Yield, moles/100 males 2 011142 fucrl 'Inmp I V 7 Sample Time 1 (.onv. Yield Mod. (J0 ()0: ("z ()3 Ci fl-Mli -l 2 -M1i-1 2 -.\113- 2 isoprr-nr- 70:30 1, 48. 5 45. 3 .13. -'1 0. 48 22.112 2. 71 10. 00 15. 31 80:00 1,150 47. 8 44. 5 03.1 0. 57 0. 55 1. 0i 3. 45 30. 15 11. 17 80:00 1,150 48. 7 45. 4 03.1 0. 40 0. 54 .2. l4 .2. 72 10.013 -15. 35 81:00 1,150 48. 8 45. 3 J2. .I 0. 58 0. 53 12.10 .2. 71 10.03 45. '25

See the following table:

2-M13--2(1HSV 400 N 2, 000 Steam (iHSV 5, 000

'lwrrstago (non-oxidativn and oxidnl-ivo) dehydrogenation First stage catalyst (iron oxide-chromium oxide-potassium carbonate); second stage catalyst (ll0lt1ill(i$1ilii11c-nxygtlii 'lvinp., e Sampll' Time 1 F. Conv. Yield Mod. Isoprcnv 58:00 1, 070 57. 0 40. 0 86. 0 1. 14 .2. 72 49. 00 58:30 1, 070 54. 4 10. 5 85. .l 0. J3 l. 05 40. 75 50:00 1,070 57.1 48.1 84. 2 1. O0 .2. 40 48.11 50:30 1,070 58. 0 -10. 0 85. 5 0. .16 12.03 50 l 'limvs are in ln's.:inin. total Linn on stream. Hydrocarbon (iIISY 400 Y Morlivity is a modilivd soli-cilvity. As nscd livrvin thv tvrms yield. ()xygon (HISY. 30h conversion, and nlotlivity" an based on analysis of tins-phase products Nitrogen. 3 for convert-0d liytll'ot'nrlmns, oxidvs of r'nrhon. :inll nnconvvrlcll fur-(l. Ste-mil. 5 000 Nov llnfollowing table:

= '1hv food for tlic sccond stage 151110 i fllnviit fi'oni thofirsl stage aftvr \vutcr rvmovnl and air and steam addition.

dehydrogenation by the two-stage (non-oxidative and oxidative) process of the invention. Both conversion and yield rates show desirable advancement when utilizing the two-stage process of the invention. These advancements were accomplished at a less severe temperature profile than could be realized before. Yet, the

total effluent of the first stage was oxidatively dehydrogenated with the iron phosphate catalyst in the second stage while maintaining the desirable low temperature profile. Certain modifications of this invention will become apparent to those skilled in the art and the illustrated details disclosed herein are not to be construed as imposing unnecessary limitation on the invention. 1

What i claim is: 1. A two-stage dehydrogenation process for producing diolefins from monoolefins, comprising:

contacting a monoolefinic feedstream in a first-stage,

non-oxidative dehydrogenation zone with a nonoxidative dehydrogenation catalyst selected from at least one of chromia or alumina, vanadia on alumina, nickel on kieselguhr, platinum on alumina, and iron oxide on an inorganic carbonate, and contacting the effluent from the first-stage, nonoxidative dehydrogenation zone with a molecular oxygen-containing gas, steam, and an ironphosphate catalyst under oxidative dehydrogenation conditions in a second stage.

2 A process according to claim 1 wherein the primary zone non-oxidative dehydrogenation temperature ranges from about 600 to about 1,300F under a pressure of about 0 to about 500 psig and the oxidative dehydrogenation zone temperature ranges from about 700 to about 1,300F under a pressure of about 0.05 to about 250 psig.

3. A process according to claim 1 wherein the oxidative dehydrogenation zone has a molecular oxygencontaining gas-to-feed ratio of from about 0.1 to about 3.0 and a steam-to-feed ratio of from about 0.1:1 to l00:l based upon gas hourly space velocity rates.

4. A process according to claim 1 wherein the ironphosphatc oxidative dehydrogenation catalyst has a phosphorus content of about 1.01 to about 5.0 times the stoichiometric amount required to react with all of the iron present wherein the phosphate is in the form of (P03 ions, and the oxygen-to-phosphorus atomic ratio is in the range of from about 3:l to about 3.999:l.

5. A process according to claim 1 wherein the firststage, non-oxidative dehydrogenation catalyst is comprised of potassium carbonate, iron oxide, and chromium oxide.

6. A process according to claim 1 wherein the monoolefinic feedstream is comprised of isoamylenes and the resulting diolefins are comprised of isoprene. 

2. A process according to claim 1 wherein the primary zone non-oxidative dehydrogenation temperature ranges from about 600* to about 1,300*F under a pressure of about 0 to about 500 psig and the oxidative dehydrogenation zone temperature ranges from about 700* to about 1,300*F under a pressure of about 0.05 to about 250 psig.
 3. A process according to claim 1 wherein the oxidative dehydrogenation zone has a molecular oxygen-containing gas-to-feed ratio of from about 0.1 to about 3.0 and a steam-to-feed ratio of from about 0.1:1 to 100:1 based upon gas hourly space velocity rates.
 4. A process according to claim 1 wherein the iron-phosphate oxidative dehydrogenation catalyst has a phosphorus content of about 1.01 to about 5.0 times the stoichiometric amount required to react with all of the iron present wherein the phosphate is in the form of (PO4 3) ions, and the oxygen-to-phosphorus atomic ratio is in the range of from about 3:1 to about 3.999:1.
 5. A process according to claim 1 wherein the first-stage, non-oxidative dehydrogenation catalyst is comprised of potassium carbonate, iron oxide, and chromium oxide.
 6. A process according to claim 1 wherein the monoolefinic feedstream is comprised of isoamylenes and the resulting diolefins are comprised of isoprene. 